1. Field of the Invention
The present invention relates to a coating liquid for use in preparing an electrophotographic photoreceptor. In addition, the present invention also relates to an electrophotographic photoreceptor prepared using the coating liquid.
2. Discussion of the Background
Inorganic photosensitive materials such as Se, CdS and ZnO have been used as photosensitive materials for electrophotographic photoreceptors for use in electrophotographic image forming apparatus. However, in recent years organic photosensitive materials are mainly used for such electrophotographic photoreceptors because of having advantages in photosensitivity, thermal stability and toxicity. Among the electrophotographic photoreceptors including an organic photosensitive material, functionally-separated photoreceptors having a configuration such that a charge generation layer and a charge transport layer are overlaid are typically used now because of having good photosensitivity and durability.
In general, electrophotographic image forming apparatus such as printers, copiers and facsimile machines perform a series of image forming processes including a charging process, a light irradiation process, a development process, a transferring process and a fixing process. In addition, recent electrophotographic image forming apparatus have been improved so as to have high speed printability and high durability. Therefore, the photoreceptors used for such image forming apparatus are required to have good reliability such that high quality images can be produced even when the photoreceptors are repeatedly used for a long period of time. In particular, super high speed copiers produce huge volume of copies. The photoreceptors used for such super high speed copiers have to be frequently replaced with new photoreceptors, resulting in deterioration of the productivity of the copiers. In tandem-type color image forming apparatus in which four developing units are arranged in parallel, a photoreceptor having a relatively small diameter is typically used therefor to prevent jumboization of the image forming apparatus. Therefore, a need exists for a photoreceptor having higher durability.
When a photoreceptor is repeatedly used for image forming apparatus using a nega-posi developing method for a long period of time, a background development problem in that the background areas of images are soiled with toner particles is often caused. Specific examples of the causes for the background development problem are as follows:    (1) contamination and defects of the electroconductive substrate used for the photoreceptor;    (2) dielectric breakdown of the photosensitive layer of the photoreceptor;    (3) injection of carriers into the photosensitive layer from the electroconductive substrate;    (4) the dark decay property of the photoreceptor deteriorates; and    (5) thermal carriers generated in the photosensitive layer.
Among these causes, the cause (1) can be removed by cleaning the substrate before forming the photosensitive layer thereon. Therefore, it is an ultimate solution to improve the voltage resistance and electrostatic fatigue property of the photoreceptor, and to prevent occurrence of carrier injection from the substrate.
From this point of view, techniques such that an undercoat layer (or an intermediate layer) is formed between an electroconductive substrate and a photosensitive layer have been proposed. For example, published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 47-6341 discloses an intermediate layer including a nitrocellulose, and JP-A 60-66258 discloses an intermediate layer including a nylon resin. In addition, JP-A 52-10138 discloses an intermediate layer including a maleic acid based resin, and JP-A 58-105155 discloses an intermediate layer including a polyvinyl alcohol resin.
However, these intermediate layers are a resin layer and have a high electric resistance. Therefore, the residual potential of the photoreceptors increases, resulting in decrease of image density when images are formed by a nega-posi developing method. In addition, such intermediate layers exhibit ionic conduction caused by impurities included therein, and therefore the electric resistance thereof increases particularly under low temperature and low humidity conditions, resulting in increase of the residual potential. Therefore, the intermediate layers have to be thinned, and thereby a problem in that the charge properties of the resultant photoreceptors deteriorate after repeated use is caused.
In attempting to solve this problem (i.e., in attempting to control the resistance of an intermediate layer), techniques in that an electroconductive material is included in an intermediate layer have been proposed. For example, JP-A 51-65942 discloses an intermediate layer in which carbon or a chalcogen based material is dispersed in a crosslinked resin. JP-A 52-82238 discloses an intermediate layer which is crosslinked using an isocyanate crosslinking agent upon application of heat thereto and which includes a quaternary ammonium salt. JP-A 55-113045 discloses a resinous intermediate layer including a resistance controlling agent. JP-A 58-93062 discloses a resinous intermediate layer including an organic metal compound. However, the photoreceptors including such a resinous intermediate layer have a drawback in that images having moiré fringes are produced when the photoreceptors are used for image forming apparatus using coherent light such as laser light for image writing.
In attempting to control the resistance of the intermediate layer while preventing occurrence of the moiré fringe problem, intermediate layers including a filler have been proposed. For example, JP-A 58-58556 discloses a resinous intermediate layer including aluminum oxide or tin oxide. JP-A 60-111255 discloses a resinous intermediate layer including a particulate electroconductive material. JP-A 59-17557 discloses an intermediate layer including magnetite. JP-A 60-32054 discloses a resinous intermediate layer including titanium oxide and tin oxide. JP-As 64-68762, 64-68763, 64-73352, 64-73353, 01-118848 and 01-118849 have disclosed resinous intermediate layers including a powder such as borides, nitrides, fluorides and oxides. In these resinous intermediate layers including a filler, the content of a filler in the intermediate layers has to be increased (i.e., the content of a resin has to be decreased) so that the resultant intermediate layers have the desired electric properties. Therefore, the adhesion of the intermediate layers to an electroconductive substrate deteriorates, and thereby a peeling problem in that the intermediate layers are separated from the electroconductive substrate tends to occur. Particularly, when the substrate is a flexible belt, the peeling problem occurs more frequently.
In attempting to solve the peeling problem, photoreceptors having a layered intermediate layer have been proposed. The proposed photoreceptors are broadly classified into two types, which have structures as illustrated in FIGS. 1 and 2. The photoreceptors of first type, which have a structure as illustrated in FIG. 1, include an electroconductive substrate 1, and a resin layer 2 including a filler, a resin layer 3 including no filler, and a photosensitive layer 4, which are overlaid in this order. The photoreceptors of second type, which have a structure as illustrated in FIG. 2, include an electroconductive substrate 1, and a resin layer 3 including no filler, a resin layer 2 including a filler and a photosensitive layer 4 which are overlaid in this order.
Specifically, in the first type photoreceptors, the electroconductive layer 2 includes a filler having a low electric resistance and is formed on the electroconductive substrate 1. In addition, the resin layer 3 is formed thereon. The intermediate layers of this type have been disclosed in JP-As 58-95351, 59-93453, 04-170552, 06-208238, 06-222600, 08-184979, 09-43886, 09-190005 and 09-288367.
In the intermediate layers of this type, the electroconductive layer 2 serves as an electrode. Therefore the intermediate layers are electrically the same as the resinous intermediate layers mentioned above, and thereby the above-mentioned electrostatic problem caused by the photoreceptors having a resinous intermediate layer cannot be solved. Since the electroconductive layer includes a filler, occurrence of moiré fringes can be prevented because the light beam for image writing scatter. When such a photoreceptor is charged, charges having a polarity opposite to that of the charges formed on the surface of the photoreceptor reach the interface between the electroconductive layer 2 and the resinous layer 3. However, when the electroconductive layer 2 has a relatively high resistance, charges are not well injected from the electroconductive substrate 1, and the resistance of the layer 2 increases after long repeated use, thereby increasing the residual potential of the photoreceptor. In addition, in order to avoid the problem caused by defects of the electroconductive substrate 1, the layer 2 has to have a thickness not less than about 10 μm. In this case, the residual potential increasing problem remarkably occurs.
JP-A 04-324450 discloses a photoreceptor having a configuration as illustrated in FIG. 1 in which the resinous layer 3 includes a polyamide resin including ions in an amount of from 0.001% to 0.02% by weight. It is described therein that the resultant photoreceptor has good environmental stability and can produce images with few black spot images.
In the photoreceptors of second type, a positive hole blocking layer is formed on the electroconductive substrate, and a resin layer including a filler having a low resistance or an electroconductive filler is formed on the positive hole blocking layer. Such a layered intermediate layer has been disclosed in JP-As 05-80572 and 06-19174. The photoreceptors of this type hardly cause the background development problem because the intermediate layer thereof has a positive hole blocking function. In addition, since a filler-containing layer is present as an upper layer, residual potential hardly increases. Specifically, injection of positive holes from the electroconductive substrate 1 to the photosensitive layer 4 can be avoided, and thereby the background development problem in a nega-posi development method is hardly caused. In addition, since a charge blocking layer is formed as a lower layer, the degree of increase of residual potential of the photoreceptor after long repeated use is lower than in the case where the charge blocking layer is formed as an upper layer.
The resins for use in such intermediate layers have to fulfill the following requirements:    (1) The intermediate layer has such a good solvent resistance as not to be easily dissolved in a solvent or to be easily deformed by the solvent when a photosensitive layer having a charge generation layer (hereinafter referred to as a CGL) and a charge transport layer (hereinafter referred to as a CTL) is formed on the intermediate layer by a wet coating method using a coating liquid including such a solvent;    (2) The intermediate layer has good electric barrier property;    (3) The intermediate layer has such good film forming property as to hardly include coating defects; and    (4) The intermediate layer has good adhesion to an electroconductive substrate.
Therefore, polyamide resins (preferably N-alkoxymethylated polyamide resins) are typically used for the intermediate layer. For example, JP-A 09-265202 discloses an undercoat layer including an alkoxymethylated nylon copolymer having an alkoxymethylation degree of from 5 to 30%. JP-A 2002-107984 discloses an intermediate layer including a crosslinked N-alkoxymethylated polyamide and an inorganic pigment. Japanese patent No. 3,086,965 (i.e., JP-A 04-330455) discloses an intermediate layer including an N-alkoxymethylated polyamide copolymer having a main unit obtained from λ-amino-n-lauric acid. In addition, Japanese patent No. 3,226,110 (i.e., JP-A 05-11483) discloses an intermediate layer including a polyamide resin having a unit having a specific formula. Thus, it is well known to use an intermediate layer (or an undercoat layer) including an N-alkoxymethylated nylon for preventing occurrence of injection of charges from an electroconductive substrate (i.e., for preventing occurrence of the background development problem).
However, the photoreceptors having a layered intermediate layer or an intermediate layer including an N-alkoxymethylated nylon tend to have a relatively high residual potential. In particular, the photoreceptors have a seriously high residual potential after long repeated use. This is a problem to be solved. In addition, an N-alkoxymethylated nylon resin having a high hygroscopic property is used for the intermediate layer, the resistance of the layer seriously changes, and the residual potential of the photoreceptors seriously increases under low temperature and low humidity conditions, resulting in occurrence of the background development problem. In addition, the photoreceptors have a low potential under high temperature and high humidity conditions, and thereby the background development problem is also caused. Namely, the photoreceptors have large environmental dependence.
In attempt to solve the problem, Japanese patent No. 2,718,044 (i.e., JP-A 01-177556) discloses an undercoat layer including an N-alkoxymethylated nylon resin including Na, Ca and P atoms as impurities, each of which is included in the resin in an amount of not higher than 10 ppm. JP-A 06-93129 discloses an intermediate layer including an N-alkoxymethylated 6-nylon including components having a molecular weight not higher than 1000 in an amount of not higher than 10 ppm. Japanese patent No. 2,887,209 (i.e., JP-A 04-240862) discloses an intermediate layer which is formed using an alcohol-soluble nylon which has been subjected to a contact treatment using a mixture solvent of an alcohol and a ketone.
These intermediate layers use an N-alkoxymethylated nylon resin which have been treated to an impurity removing treatment to improve the electrostatic property of the resultant photoreceptors. However, the intermediate layer disclosed in Japanese patent No. 2,718,044 can avoid the residual potential increasing problem under high temperature and high humidity conditions but cannot solve the residual potential increasing problem under low temperature and low humidity conditions because the resistance of the intermediate layer seriously increases under low temperature and low humidity conditions. The photoreceptor disclosed in JP-A 06-93129 has good stability at room temperature, but causes the background development problem under high temperature and high humidity conditions and low temperature and low humidity conditions due to decrease of the potential or increase of the residual potential. The photoreceptor disclosed in Japanese patent No. 2,887,209 has good potential contrast even under high humidity conditions and low temperature and low humidity conditions, but the long-term durability of the photoreceptor is not satisfactory. Therefore, the photoreceptor cannot be used for recent high speed electrophotographic image forming apparatus using a photoreceptor having a small diameter.
In addition, N-alkoxymethylated nylon resins are soluble in only alcoholic solvents with minor exceptions. Therefore, a coating liquid including an N-alkoxymethylated nylon resin and an alcoholic solvent is typically used for forming an intermediate layer. However, such coating liquid tends to cause a problem in that the coating liquid becomes clouded after long preservation particularly under low temperature conditions. If such a coating liquid is coated, coating defects are formed in the resultant intermediate layer. This is not described in the above-mentioned patents and publications.
JP-A 09-152731 discloses a technique in that by adding a halogenated hydrocarbon to an alcoholic intermediate layer coating liquid, the long-term preservability of the coating liquid can be improved. However, it is not preferable to use a halogenated hydrocarbon in view of environmental protection, and therefore the method cannot be practically used.
In addition, JP-A 2000-56494 discloses a technique in that benzyl alcohol is added to an alcoholic intermediate layer coating liquid. By using this technique, the long-term preservability of the coating liquid can be improved. However, it takes a long time to dry the coating liquid (i.e., it takes a long time until the coated layer achieves a finger-touchable drying condition), and thereby coating defects tend to be caused in the coated layer. In addition, it is necessary to dry the coated layer at a relatively high temperature because benzyl alcohol has a high boiling temperature.
Because of these reasons, a need exists for a photoreceptor which has an intermediate layer without coating defects and which has such a good charge stability as to be able to produce high quality images without causing the background development problem even when the environmental conditions are changed.